The present invention relates to a control signal receiving method and system for use in receiving a control signal in, for example, a cellular mobile telecommunication system in which the control signal is transmitted in an idle period during which message is absent and, particularly, to such method and system by which a power consumption during the idle period is minimized.
A mobile telecommunication system in which a service area is divided into a plurality of cells and a common frequency is used for all of the cells is known as the cellular mobile telecommunication system. In this system, a telephone communication between a stationary telephone set and a mobile station mounted on a vehicle in the service area is made possible through a land station provided for each cell and the land station exchanges control signals with the mobile station to monitor a location of the mobile station so that the communication between the stationary set and the mobile station can be performed under an optimal condition.
A construction of the mobile station to be used in such cellular telecommunication system is shown in FIG. 1. As shown in FIG. 1, the mobile station comprises an antenna 1, an antenna duplexer 2 connected to the antenna 1, a receiver section 3 connected to the antenna duplexer 2, a PLL synthesizer section 4, a transmitter section 5, a data modem/ control section 6 and a telephone set 7. A circuit construction of the PLL synthesizer section 4 is shown in FIG. 2.
In FIG. 1, a message signal (audio signal) transmitted from a stationary telephone set through a land station during a talking period is received by the antenna 1 and fed through the antenna duplexer 2 to the receiver section 3. In the receiver section 3, the signal is converted by an output of the PLL synthesizer section 4 into an IF signal which is sent to the telephone set 7 after being detected. On the other hand, an audio signal produced by the telephone set 7 is fed to the transmitter section 5 and, after modulation by the data modem/control section 6, converted by an output of the PLL synthesizer section 4 into a transmission frequency which is transmitted through the antenna duplexer 2 and the antenna 1.
In the audio communication performed in this manner, a control signal is exchanged between the land station and the mobile station during the idle period where audio signal is absent. The control signal from the land station may include any one or more of data signals including a location information for identifying a cell where the mobile station locates a channel control signal for informing a control channel in that cell, an interrogation signal to be received by a mobile station assigned by a stationary set and a channel assigning signal for assigning a communication channel to a mobile station, etc. The control signal to be sent by the mobile station may include any one or more of data signals including a response signal to an interrogation from the land station, an interrogation signal for interrogating a stationary set and a location registration signal for informing the land station of a cell where the mobile station is currently located, etc.
The control signal received by the mobile station is converted in the receiver section 3 into the IF signal and fed to the control section 6 for demodulation in the same way as that for the audio signal.
In response to the control signal from the land station or when an audio communication is to be started, a data signal such as the response signal, the interrogation signal and/or the location signal is formed in the control section 6 and it is modulated to form the control signal. The control signal thus formed is modulated, frequency-converted and then transmitted through the antenna 1.
These operations are performed under the control of the control section 6. The PLL synthesizer section 4 functions to convert the received signal into an IF signal of a specific frequency and to provide a signal having a frequency necessary to convert a signal to be transmitted thereby into a signal having a frequency assigned thereto by the land station as mentioned previously. The number of the control channels for the control signal transmission and the number of channels necessary for audio communication are usually very large and, for that reason, it is necessary to change the output frequency of the PLL synthesizer section 4 according to the channel to be used. This is performed under the control of the control section 6.
The control signal to be transmitted by the land station is typically composed of a repetition of signal frames each including a bit synchronizing signal of, for example, 10 bits, a word synchronizing signal of 11 bits and a data stream portion in the order, as shown in FIG. 3. The data stream portion is composed of a group A of data streams A1, A2, A3, A4 and A5 and a group B of data streams B1, B2, B3, B4 and B5, for example. The data stream groups A and B are arranged alternately as shown. The data streams of the group A contain identical data and those of the group B contain identical data different from that of the Group A.
Each data stream is of 40 bits and a busy/idle (B/I) information of 1 bit is inserted into a place preceding the bit synchronizing signal and a place preceding the word synchronizing signal, respectively. Four of such B/I informations are inserted into each data stream, as shown by arrows in FIG. 3. Therefore, one frame is composed of 463 bits. A transmission rate of this control signal is usually 10K bits/sec and therefore 1 frame transmission requires 46.3 msec and 1 data signal transmission requires about 4 msec.
As mentioned previously, there are a plurality of control channels for each cell. Therefore, when a mobile station enters into a new cell, the PLL synthesizer section 4 thereof changes its output frequency sequentially under the control of the control section 6 to perform a frequency scanning to find out a control channel whose field intensity is maximum, by detecting the receiving level of the control signal. When an optimum control channel is found, the location of the mobile station is registered for that channel and a subsequent transmission of the control signal is performed by using that channel.
Therefore, in order to obtain a best receiving condition in the selected channel, an output frequency of the PLL synthesizer section 4 has to be maintained at a value corresponding to the optimum channel. In order to provide this condition, the control section 6 monitors the signal level of the receiving control signal during the idle time period in which there is neither interrogation nor registration of location to control the PLL synthesizer section 4 so that the control signal in the selected control channel can be received with maximum field intensity.
In the case of the control signal shown in FIG. 3, it is enough for the control section 6 to monitor either the data stream A or B. Assuming that the data stream A is to be monitored, the monitoring can be performed intermittently. In such case, the output frequency of the PLL synthesizer section 4 is controlled such that the signal level of, for example, the stream A1 becomes a maximum. With such intermittent reception of the stream A1, it is possible to minimize a power consumption of the mobile set.
As shown in FIG. 2, the PLL synthesizer section 4 comprises a voltage controlled oscillator (VCO) 13, a frequency divider stage 20 for dividing an output frequency of the VCO 13, a phase comparator 12 for comparing a phase of an output signal of the frequency divider stage 20 with a reference phase 22 and a charge pump 14 including a capacitor for holding a phase difference signal from the comparator 12, the output frequency of the voltage controlled oscillator 13 being controlled according to an output voltage of the charge pump 14. The output of the VCO 13 corresponds to the output of the PLL synthesizer section 4 and the output frequency of the VCO 13 can be changed by changing a dividing ratio of the frequency divider stage 20 by means of the control section 6.
In such PLL synthesizer section 4, the frequency divider stage 20 comprises a prescaler 11 having a pair of dividing ratios which are selectable and a variable frequency divider 10 whose dividing ratio can be controlled by the control section 6. The use of such prescaler 11 is due to the facts that the output frequency of the VCO 13 must be divided substantially and that the output frequency as high as, for example, 800 MHz of the VCO 13 requires a very high dividing rate. Since, when the prescaler 11 is used, a large current flows through the prescaler necessarily for the reason mentioned above, the power consumption thereof becomes considerable.
In order to minimize such large power consumption of the prescaler, Japanese Patent Application Laid-Open No. 157142/1981 proposes a system in which, during reception the data stream of the group A, power is supplied to the prescaler to make it operative and, during reception the data stream of the group B, the power supply to the prescaler is terminated. Although there is no phase difference signal provided by the phase comparator at a time period in which no power is supplied to the prescaler, the operation of the VCO can be maintained by a phase difference signal stored in the charge pump. Since the time period in which the prescaler consumes power is reduced in this manner, a reduction of power consumption is realized.
When the prescaler is deenergized, the output frequency of the VCO reduces gradually with time, a reduction rate being determined by a discharge time constant of the charge pump which can be selected such that the output frequency does not affect the signal receiving operation substantially for a desired time. However, when the prescaler is energized, there is a transient phenomenon of the VCO upon which the output frequency of the PLL synthesizer section fluctuates substantially, causing a monitoring of the necessary data stream to be impossible for at least such fluctuating time.